Bioprospecting and Repurposing of Leaf Biomass to Support Sustainable Biopharmacy: Evaluation of Seasonal Chemical Variations and Biological Activities of Six Eucalyptus Essential Oils

This study focuses on six Eucalyptus species, namely E. badjensis, E. benthamii, E. dunnii, E. grandis, E. globulus, and E. saligna aiming at a sustainable use of their green biomass. These species were grown at the same location and stage of development. Leaf essential oils were extracted by hydrodistillation using a Clevenger apparatus during the four seasons. Gas chromatography–mass spectrometry (GC–MS) and high-performance thin-layer chromatography (HPTLC) techniques were used to analyze the chemical composition. E. badjensis consistently exhibited a dominant composition, with 1,8-cineole being the predominant component. Notably, the proportion of 1,8-cineole in E. badjensis was 77.35% in spring (SP), 69.46% in summer (SU), 95.30% in autumn (AU), and 89.30% in winter (WI). E. globulus also exhibited 1,8-cineole as its primary constituent, with proportions fluctuating slightly across seasons at 84.87%, 79.94%, 81%, and 85.88%, respectively. The proportions and constituents of various species differed significantly. HPTLC was successfully used as a swift technique to monitor the chemical composition of essential oils (EOs) in various Eucalyptus species during the seasonality. GC–MS and HPTLC analysis showed that different Eucalyptus species displayed unique chemical compositions, while both the chemical profile and productivity of all analyzed EOs were affected by seasonality. This finding was demonstrated in the principal components cluster. The analysis of six species has revealed that the EO of E. benthamii is the most potent in thwarting the infestation of Cimex lectularius. Additionally, all six EOs demonstrated antioxidant activity in the ABTS model. EOs of E. benthamii, E. dunnii, and E. grandis showed significant anti-inflammatory activities in experimental models.